This invention relates to a composition for coating glass fiber articles. More particularly, it relates to a silicone coating composition having improved curing properties, capable of forming a cured film having improved flexibility, electrical insulating properties, and heat resistance, and suitable for the treatment of glass fiber articles such as glass sleeve, glass cloth and glass tape.
In these years, most electric appliances are required to be more multifunctional and compact, and electrical insulating materials for such electric appliances are often used in complicated bent shapes or under various environments including a high temperature environment near heating elements or a high humidity environment. There is a need for electrical insulating materials capable of maintaining good electrical insulating properties upon exposure to severe environments as mentioned above while exhibiting excellent flexibility and heat resistance.
Typical of conventional electrical insulating materials are glass fiber articles in the form of a glass sleeve, glass cloth or glass tape coated with organopolysiloxane compositions. Exemplary well-known organopolysiloxane compositions are those comprising an organopolysiloxane having a monovinyldiorganosilyl radical at both ends thereof, an organohydrogenpolysiloxane containing a hydrogen atom attached to a silicon atom, and a platinum curing catalyst as disclosed in Japanese Patent Publication Nos. 51-37399, 51-46880 and 53-13505, and Japanese Patent Application Kokai Nos. 52-63495 and 55-154354.
These organopolysiloxane compositions crosslink and cure through relatively brief heating into flexible coatings having electrical insulation and heat resistance. These films, however, do not perform well under severe conditions as described above.
Various attempts have been made on the above-mentioned organopolysiloxane compositions to produce more flexible coatings, for example, by increasing the molecular weight of vinyl radical-containing organopolysiloxane in the composition, or decreasing the content of vinyl radical in the organopolysiloxane or active hydrogen in organohydrogenpolysiloxane to reduce the degree of crosslinking of the entire composition. This method, however, adversely affects curing of organopolysiloxane compositions, which suffer from many troubles due to undercure, including formation of a tacky coating by curing under usual heating conditions, or a cured coating having substantially reduced strength and dielectric properties. Extended heating at a higher temperature also leads to a loss of productivity.
It is common practice to color electrical insulating materials for the purpose of identifying wires and parts. Many pigments and dyes used as colorants are poisons to platinum catalysts. The addition of a colorant to an organopolysiloxane composition as formulated above invites some problems in that curing is retarded and the resulting coatings exhibit poor dielectric properties. The deleterious effect of the catalyst poison is more serious with those compositions wherein the vinyl radical content of organopolysiloxane or the active hydrogen radical content of organohydrogenpolysiloxane is reduced to improve the flexibility of the resulting coatings. When the amount of platinum catalyst is increased to compensate for a loss of curing, not only is the coating composition undesirably increased in cost because of the expensive platinum catalyst, but it has a reduced pot life because of a viscosity rise and gelation. It has been quite difficult to find a good compromise between coating flexibility and poisoning resistance.